The present invention relates to a method for increasing active duration time of an automatic freewheeling function in a vehicle with a combustion engine, a transmission and a cruise control.
The present invention also relates to a computer program and computer program product both to be used with a computer for executing said method.
An automatic vehicle transmission designed from planetary gears usually has one-way couplings between the planetary gear steps, which, when a propulsion engine is in drive, lock in the automatic gear position for torque transmission from the engine to the drive wheels, but which, when torque is transmitted in the opposite direction, i.e. at zero throttle and with the vehicle in motion, disengage and cause the vehicle to freewheel without engine braking, which, by utilizing the kinetic energy of the vehicle, yields lower fuel consumption than if the engine is busy braking.
A corresponding freewheel function for automated mechanical transmissions (AMT) is shown in WO 2002/092378. Here, a freewheel function is obtained by putting a splitter gear disposed in the gearbox into its neutral position when an accelerator pedal disposed in the vehicle is located within a predetermined swivel angle range, which starts at a distance from the rest position of the pedal and extends over a slight angle within the total swivel angle of the pedal. When the driver wishes to change the driving condition from driving engine to allowing the vehicle to freewheel without engine braking, he lets up the accelerator pedal, so that it reaches the predetermined range when the drive from the engine is broken. If engine braking is required, the driver further releases the accelerator pedal, either fully or, in any event, to the point where it passes through the predetermined range within which disengagement occurs.
Another example of a prior art freewheel function is disclosed in WO 2005/084995. The freewheel function is automatically activated when a predetermined vehicle condition is prevailing. This condition comprises at least traveling in a not too steep downslope and that there is no fuel demand from the driver (via the accelerator pedal) or from a cruise control in the vehicle. When the freewheel function is activated with a certain vehicle set speed, i.e. during a time period when the cruise control is active, and a certain over speed limit (VBCC set speed) is exceeded, then the freewheel function is automatically inactivated and the vehicle is automatically braked to a speed under said VBCC set speed. When vehicle speed is below VBCC set speed the freewheel function will be activated again if the right vehicle conditions are still prevailing.
WO2005/084995 discloses that said VBCC set speed can be automatically adjusted in dependence of at least prevailing road gradient.
WO2005/084995 further discloses that said VBCC set speed can be automatically adjusted in dependence of road gradient in the near future, e.g, in dependence of that the prevailing downhill slope will end in a near future, which means increased VBCC set speed in order to postpone inactivation of the freewheel function.
In certain downslopes where the vehicle travel resistance is approximately zero, arrangements according to prior art will not activate the freewheel function since a too short freewheel duration time is foreseen (calculated) for a certain vehicle set speed, thus an arrangement according to prior art needs a downslope with less travel resistance (in practice a steeper down slope) to initiate activation of the freewheel function.
The freewheel function cannot be activated during a too short duration time, due to fuel punishment. Another reason not to activate the freewheel function during a too short duration time is that travel comfort otherwise would decrease. In prior art arrangements hysteresis is used when travel resistance is approximately zero, which means that the vehicle needs to be accelerated (preferably without fuel injection) to a vehicle speed a bit above a vehicle set speed, before the freewheel function can be allowed to be reactivated. This will secure enough freewheel duration time.
It is desirable to solve the problem of how to extend the freewheel duration time further, so that fuel savings can be increased even further and without decreased travel comfort.
The method according to an aspect of the invention is a method for increased freewheel active duration time during a cruise control active time period. With an increased freewheel duration time more fuel will be saved. The invention is especially beneficial/effective in downslopes with a relatively small inclination, i.e. where vehicle travel resistance is approximately zero.
The above mentioned problem is solved by manipulating the speed area within which the freewheel function is allowed to be activated. This can be done by a method where, said function comprises (includes, but is not necessarily limited to) means for determining a first vehicle set speed for when said function is allowed to be activated under at least prevailing conditions. This method is according to an aspect of the invention characterized in the steps of:                calculating a predetermined allowable vehicle speed drop to an under speed value below said first vehicle set speed for at least prevailing conditions and;        controlling said function based on said under speed value, in order to extend active duration time of said function.        
Thus, the freewheel function duration time is extended by allowing a certain predetermined vehicle speed drop below said first vehicle set speed before the freewheel function is controlled. In one embodiment of the invention said controlling comprises the step of inactivating said freewheel function when vehicle speed has decreased down to said first under speed value from a vehicle speed above said under speed value. In another embodiment of the invention said controlling comprises the step of activating said freewheel function when vehicle speed has increased up to said first under speed value from a vehicle speed below said under speed value. Thus, if vehicle speed is approaching the under speed value through retardation and the freewheel function is currently activated, it will be inactivated when the under speed value is reached. On the other hand if vehicle speed is approaching the under speed value through acceleration, i.e. the freewheel function is currently inactive, it will be activated when the under speed value is reached.
In one embodiment of the invention the same effect can be achieved by simply move the first vehicle set speed to a second vehicle set speed with a lower vehicle speed value, thus said second vehicle set speed is equal in functionality to said vehicle under speed value.
In one embodiment of the invention the determination of the magnitude of the allowed predetermined vehicle speed drop to said under speed value below said first vehicle set speed is dependent on the vehicle's ability to accelerate up to said first vehicle set speed from said under speed value within a reasonable predetermined time. Thus, the allowed vehicle speed drop can be adapted to prevailing vehicle load and available propulsion power.
In another embodiment of the invention said predetermined vehicle speed drop below said first vehicle set speed will not occur if the vehicle is expected to meet increased travel resistance, for example an uphill slope, in the near future. The advantage of this embodiment is a better uphold of vehicle average speed and under most circumstances also a lower total fuel consumption.
In a further embodiment of the invention the magnitude of said vehicle under speed value will not be lower than that of a highest gear of a gearbox in the vehicle, or a gear engaged just before the freewheel function was activated, will be possible to reengage when the vehicle speed reaches said vehicle under speed value and the freewheel function will be inactivated and a gear will have to be engaged.
In a further embodiment of the invention the vehicle under speed value can be manually set for at least one first vehicle set speed by the driver.
In another embodiment of the invention the vehicle under speed value can be set for one or several different first vehicle set speeds by a manufacturer of the vehicle or authorized workshop personnel.
In a further embodiment of the invention when the vehicle is driven with a vehicle speed below said vehicle under speed value and the freewheel function is currently inactivated, the freewheel function will be activated when said vehicle under speed is reached due to vehicle acceleration through the force of gravity. This can happen for example when the vehicle passes a crest.